Resistance material and method of making

ABSTRACT

Lightning-arrester resistance material is composed of silicon carbide particles and a binder therefor composed of cordierite formed of a fired mixture of cordierite-forming glass material and cordierite-forming crystalline material.

United States Patent John J. Pitha Lenox, Mass.

Dec. 5, 1968 Sept. 21, 1971 General Electric Company Inventor Appl. No.Filed Patented Assignee RESISTANCE MATERIAL AND METHOD OF MAKING 9Claims, 1 Drawing Fig.

Int. Cl 1101b 1/04, C04b 35/ 14 Field of Search 252/516,

VO LTS [56] References Cited UNITED STATES PATENTS 3,291,759 12/1966Pitha 252/516 3,480,452 1.1/1969 Fleischner Primary ExaminerDouglas J.Drummond Attorneys-Sidney Greenberg, Francis X. Doyle, Vale P.

Myles and Frank Neuhauser ABSTRACT: Lightning-arrester resistancematerial is composed of silicon carbide particles and a binder thereforcom-' posed of cordierite formed of a tired mixture of cordieriteformingglass material and cordierite-forming crystalline material.

l I i 10 100 1000 AMPERES PATENTEU SEP2I |97l "346070.790

RESISTANCE MATERIAL AND METHOD OF MAKING The present invention relatesto resistance material, and more particularly to resistance material foruse with electric discharge devices, such as lightning arresters, andthe method of making the resistance material. Such material, also knownas nonlinear resistance or valve element material, is of the type havingvariable resistance characteristics, so that when placed in electricalcircuit with a source of electrical potential applied thereto, itsresistance decreases with an increase in the electrical potential. inknown types of overvoltage protective devices such as lightningarresters, a gap structure is usually arranged in series with theresistance material, and when the 7 device is subjected to overvoltage,such as caused by lightning or a switching surge,'the gap arcs over andwith the nonlinear resistance material forms a low-resistance path toground. The resistance material provides a low-resistance path to highvoltages and a high-resistance path to low voltages. When theovervoltage surge has been discharged, the resistance material providesa high-resistance path to the the power follow current, limiting suchcurrent to enable the gap structure to interrupt the current and returnthe arrester to its open-circuit condition.

Resistance material of the above-described type comprising a mixture ofsilicon carbide particles and a binder material therefor comprisingcordierite is disclosed in U.S. Pat. No. 3,291,759-Pitha, granted Dec.13, 1966. The present invention is an improvement on the material andmethod of the latter patent.

It is an object of the invention to provide improved nonlinearresistance material of the above-described type and a method of makingthe same.

Another object of the invention is to provide non linear resistancematerial ofthe above type having improved resistivity conditions whileproviding reduced power follow currents.

A particular object of the invention is the provision of an improved'non linear resistance material comprising silicon carbide particles anda cordierite binder therefor.

Still another object of the invention is to provide an improvedcomposition and method for forming cordierite, and for making nonlinearresistance material incorporating such codierite-forming composition.

Other objects and advantages will become apparent from the followingdescription and the appended claims.

With the above objects in view, the present invention in one of itsaspects relates to a method of making nonlinear resistance materialcomprising the steps of mixing silicon carbide particles with acordierite-forming glass material and cordierite-forming crystallinematerial, compacting the mixture, and firing the thus-treated mixture atelevated temperature for reacting the crystalline material and forforming a cordierite binder for the silicon carbide particles from themixture of glass material and crystalline material.

The invention will be better understood from the following descriptiontaken in conjunction with the accompanying drawing, in which:

The single FIGURE is a graph showing a comparison of the volt-amperecharacteristics of the resistance material of the present invention andthose of prior art resistance material.

In the aforementioned Pitha patent, there is disclosed a nonlinearresistance material formed of a mixture of silicon carbide particles anda cordierite binder therefor, wherein the binder material is obtained byfiring a mixture of appropriate proportions of talc and a porcelainmixture including flint, clay and soda or potash spar.

in accordance with the present invention an improved resistance materialis made by including in the initial binder mixture a suitable proportionof cordierite-forming glass. By cordierite-forming glass" as used hereinis meant a noncrystalline solid material having the composition 2MgO-2AlO SSiO and which when heated to elevated temperature below the meltingpoint of the glass and gradually cooled will form a ceramic composed ofcordierite. The cordierite-forming glass used in the present invention(also referred to herein simply as cordierite glass) is thusdistinguished from the aforementioned mixture of crystalline compoundsused in the prior process to form the cordierite binder in the firedproduct. As indicated, however, .both types of cordierite-formingmaterials are used in the present invention to obtain nonlinearresistance material of optimum properties.

By virtue of the addition of cordierite glass to the-binderformingmaterial in accordance with the invention, the resultant resistancematerial retains all the advantages of the prior material made with onlycrystalline cordierite-forming compounds, such as low resistivity toovervoltages and high capacity to withstand repeated surges of current,and, in addition it is characterized by a substantially higherresistance in the low voltage range than the prior resistance material.As a result, the improved material when used, for example, in lightningarresters, more effectively cuts out power follow currents and thusenables more rapid extinguishing of the are formed by overvoltages inthe arrester, so that the electrical apparatus protected by the arresterresumes normal operation in a shorter time. An additional advantageobtained by such properties of the non linear resistance material isthat the life of the lightning arrester is thereby substantiallyprolonged.

The FIGURE graphically illustrates the comparative voltamperecharacteristics of the prior and present nonlinear resistance materials.In the graph, in which the volts and amperes are plotted in logarithmicscale, Curve B represents the prior material as made by the processdescribed in the aforementioned Pitha patent, while Curve A representsresistance material of the present invention. As is evident, at thelower voltage levels, e.g., below l,000 volts, the material representedby Curve A exhibits substantially higher resistivity than the Curve Bmaterial, while at voltages from 2,000 volts and higher, theresistivities of the respective materials are equivalent, and thus theyprovide equivalent protection under high overvoltage conditions. Theincreased resistivity of the improved resistance material at the lowervoltage levels as shown is as much as 58 percent in terms of voltagerequired for passage of l ampere of current.

In a typical process of making the improved resistance material thefollowing preferred composition in percent by weight has been found toproduce satisfactory results for use in DC lightning arresters: a

Silicon carbide 75% Cordieritc glass I092 Talc 61 Porcelain mixture 9 1As well understood by those versed in the art, the porcelain mixture maycomprise flint, clay and soda or potash spar, but any particular type orcomposition of porcelain mixture may be utilized in practicing thepresent invention.

The cordierite glass used in the above mixture is made by mixing thefollowing ingredients in the typical composition shown in percent byweight:

sio, 5 1.3 1 Ano, 34.9% MgO I18 I,

This mixture is heated to fusion in an electric arc furnace at atemperature no lower than about 1,800 C. causing the reaction of theingredients and forming a molten glass material which is then quenchedto prevent devitrification. as a result of which the material solidifiesas a glass. The solid glass is thereafter ground into finely dividedform for use in the binder mixture described above. When such glass istired at elevated temperature below the melting point of the glass andthereafter cooled, it devitrifies (crystallizes) into a hard, denseceramic material consisting of cordierite, which has the formula2Mg0'2Al- 0 '5SiO,-

The remaining binder ingredients, viz, talc and porcelain mixture, arealso capable of forming cordierite when heated to elevated temperature,but in contrast to the prereacted cordierite-forming glass component,are crystalline in nature and are employed in this form in the prefiredmixture.

After all of the described components in finely divided form are mixedtogether and a suitable temporary binder such as water is added, themixture is pressed into discsor otherwise formed into desired shapes,and in such form the material is fired at a temperature of from aboutl,200 C. to about l,300

C. in a hydrogen atmosphere, removed from the furnace and ranges, inpercent by weight:

Silicon carbide 55.0 to 80.0% Cordierite glass 6.0 to 22.5% Talc 4.0 to27.0%

Porcelain mixture An example of a suitable porcelain mixture which maybe employed in the foregoing compositions is as follows, in percent byweight:

Flint 14.3% Clay 47.6% Soda or potash spar 38.]

Preferably, the talc and porcelain mixture components of the crystallinebinder material vary in the range of 40 to 60 percent by weight of talcand 60 to 40 percent of porcelain mixture Theporcelain mixture may bereplaced by a china clay such as Florida kaolin, and in the bindermixture equal parts by weight of such kaolin and talc may typically beused as the cordierite-forming crystalline ingredients.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

Whatl claim as new and desire to secure by letters Patent of the UnitedStates is:

1. A composition for making nonlinear resistance material consistingessentially of a mixture of silicon carbide particles with a bindermaterial having a composition comprising a mixture of cordierite-formingglass material and cordierite-forming crystalline material. 7

2. A composition as defined in claim 1, said composition consistingessentially of, in percent by weight, about 6.0 to about 22.5 percent ofsaid glass material, about 8.0 toabout 46.0 percent of said crystallinematerial, and about 55.0 to about80.0 percent of said silicon carbideparticles. h

3. A composition as defined inclaim 2, said crystalline materialconsisting essentially of, in percent by weight, about 40 to about 60percent talc and about 60 to about 40 percent porcelainmixture. V r

4. A method of making nonlinear resistancematerial comprising the stepsof providing a mixture as defined in claim 1, firing said mixture atelevated temperature for reacting said crystalline material, and coolingthe thus-fired mixture for forming a cordierite binder for said siliconcarbide particles from said mixture of glass material and crystallinematerial.

5. A method as defined in claim 4, including the step of compacting saidmixture of silicon carbide particles and said cordierite-formingmaterialpriorto said firing step.

6. A method as defined m claim 5, said e evated temperature being in therange of about 1,200 C. to about l,300 C.

7 A method as defined in claim 6, said compacted mixture being fired ina hydrogen atmosphere.

8. A nonlinear resistance material having improved low voltageresistance characteristics coupled with a high capacity for withstandingrepeated current surges for use in overvoltage protective devices, saidmaterial being made by the process defined in claim 4.

9. A nonlinear resistance material having improved low voltageresistance characteristics coupled with a high capacity for withstandingrepeated current surges for use in overvolt-i age protective devices,said material being made by the process defined in claim 6. a

Dated September 21, 1971 3i60'L79O Patent No.

John J. Pitha Invent0r(s).

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 1, 11 me 35, after "resistivity" insert characteristics for passingcurrent during overvoltage line 43, correct the spelling of "cordierite"Signed and sealed this 29th day of February 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER, JR.

Commissioner of Patents Attesting Officer

2. A composition as defined in claim 1, said composition consistingessentially of, in percent by weight, about 6.0 to about 22.5 percent Ofsaid glass material, about 8.0 to about 46.0 percent of said crystallinematerial, and about 55.0 to about 80.0 percent of said silicon carbideparticles.
 3. A composition as defined in claim 2, said crystallinematerial consisting essentially of, in percent by weight, about 40 toabout 60 percent talc and about 60 to about 40 percent porcelainmixture.
 4. A method of making nonlinear resistance material comprisingthe steps of providing a mixture as defined in claim 1, firing saidmixture at elevated temperature for reacting said crystalline material,and cooling the thus-fired mixture for forming a cordierite binder forsaid silicon carbide particles from said mixture of glass material andcrystalline material.
 5. A method as defined in claim 4, including thestep of compacting said mixture of silicon carbide particles and saidcordierite-forming material prior to said firing step.
 6. A method asdefined in claim 5, said elevated temperature being in the range ofabout 1,200* C. to about 1,300* C. 7 . A method as defined in claim 6,said compacted mixture being fired in a hydrogen atmosphere.
 8. Anonlinear resistance material having improved low voltage resistancecharacteristics coupled with a high capacity for withstanding repeatedcurrent surges for use in overvoltage protective devices, said materialbeing made by the process defined in claim
 4. 9. A nonlinear resistancematerial having improved low voltage resistance characteristics coupledwith a high capacity for withstanding repeated current surges for use inovervoltage protective devices, said material being made by the processdefined in claim 6.